A mobile communication network can transmit data to a mobile station over the radio interface using a point-to-point (p-t-p) transmission or a point-to-multipoint (p-t-m) transmission.
A p-t-p transmission is a single transmission that is addressed to a unique mobile station and that can be received only by this unique mobile station. FIG. 1 illustrates in an upper part such a point-to-point transmission from a network NW to a mobile station MS. A p-t-m transmission, in contrast, is a single transmission that is addressed to a plurality of mobile stations and that can be received by this plurality of mobile stations. FIG. 1 illustrates in a lower part such a point-to-multipoint transmission from a network NW to a plurality of mobile stations MS. In the following, a p-t-p channel refers to a radio channel on which a p-t-p transmission is carried out, and a p-t-m channel refers to a radio channel on which p-t-m transmission is carried out. Similarly, a p-t-p bearer refers to a radio bearer which is used on a p-t-p channel, and a p-t-m bearer refers to a radio bearer which is used on a p-t-m channel.
For Multimedia Broadcast and Multicast Services (MBMS), for example, it is planned to allow MBMS data transmissions via p-t-p channels and/or p-t-m channels. It is further planned, that a switch between a point-to-point channel and a point-to-multipoint channel is enabled during an ongoing data transfer.
FIG. 2a is a table presenting an exhaustive list of the bearer changes that are expected to occur for MBMS services within a cell, and FIG. 2b is a table presenting an exhaustive list of the bearer changes that are expected to occur for MBMS services after cell change. Both tables were taken from the document 3GPP TSG GERAN2#14bis, G2-030374, “MBMS bearer changes”, Nokia, San Diego, Calif., USA, 19-23 May, 2003. P-t-p to p-t-p and p-t-m to p-t-m changes are required within a cell and after a cell change due to possibly required reconfigurations of the physical resources. Within a cell, p-t-p to p-t-m changes are carried out only if n p-t-p bearers consume significantly more radio resources than one p-t-m. The default assumption is the usage of p-t-m anyhow. P-t-m to p-t-p changes are carried out only if a non-MBMS service is established in parallel to the MBMS service. After a cell change, p-t-p to p-t-m changes are carried out only if p-t-m is already established in the new cell. P-t-m to p-t-p changes are carried out only if p-t-m is not already established in the new cell. This requires the mobile station to notify itself to the network.
It is an assumption in on-going discussions on MBMS that for allowing a switch between a p-t-p bearer and a p-t-m bearer during an active multicast session, i.e. while data transfer is on-going, some synchronization is needed between the p-t-p bearer and the p-t-m bearer so that the application in the mobile station can continue running normally after the bearer change. A prerequisite for this is the tolerance of the application to the data interruption, which may be lossless or lossy, caused by the bearer change. While a lossless data interruption causes basically no problem in case of background traffic apart from taking more time to receive the payload, it could affect the user perception in case of streaming traffic when the interruption lasts longer than the duration corresponding to the amount of data in the application buffer. The user may experience, for example, a black screen, a freeze in a video presentation or silence in an audio presentation. It has to be noted, however, that it is acknowledged that bearer changes should be avoided during an active session.
While the necessity of supporting a switch from a p-t-p channel to a p-t-m channel is not clear yet, the reverse switch from a p-t-m channel to a p-t-p channel has to be supported. It is assumed that for a parallel support of MBMS and non-MBMS services, the MBMS service should be provided to the mobile station via a p-t-p connection. Thus, the establishment of a non-MBMS service for a particular mobile station monitoring one or more given MBMS service on the p-t-m channel may lead, depending on the capabilities of the mobile station, to the establishment of an MBMS p-t-p connection once the non-MBMS service is established, in order to ensure MBMS service continuity.
A p-t-p link can be optimized individually for each mobile station by means of link adaptation and power control. It is a characteristic of a p-t-m channel, on the contrary, that the link is not optimized individually for each mobile station. For example, no link adaptation occurs, and the signal power has to be sufficient to address all mobile stations monitoring the channel.
The resulting discrepancy in service between p-t-p transmissions and p-t-m transmissions is illustrated in FIGS. 3a and 3b. FIG. 3a is a diagram showing an area A in which a mobile station may acquire p-t-p and p-t-m channels in case of a good coverage and/or a high link quality. FIG. 3b is a diagram showing a first area A corresponding to the area A of FIG. 3a. Due to the enabled link adaptation and power control, a mobile station may acquire p-t-p channels throughout this area A even in case of a weak coverage and/or a bad link quality. FIG. 3b further shows a second area B, which is considerably smaller than the first area A and which lies within the first area A. In case of a weak coverage and a bad link quality in the area outside of B, a mobile station is only able to acquire p-t-p channels, while in reduced area B, both p-t-p and p-t-m channels are available.
This raises a problem which has not been dealt with so far. P-t-m data may be lost, when the radio link quality drops below the minimum quality required at the mobile station for acquiring the p-t-m channel. In case a mobile station is acquiring data for a given service over p-t-m and the radio conditions weakens, the mobile station may not be able to get the data through anymore.